1. Field of the Invention
The invention relates to pumps. More particularly, the invention relates to a centrifugal pump driven by an AC powered permanent magnet synchronous motor.
2. State of the Art
Centrifugal pumps utilize an impeller housed in a volute. Fluid enters the volute either axially or tangentially relative to the impeller and exits either axially or tangentially depending on the direction of impeller rotation. Centrifugal pumps may be driven by an AC motor or a DC motor. In either case, the pump impeller may be magnetically coupled to the rotor of the motor so that the workings of the motor can be hermetically sealed and isolated from the fluids passing through the pump. In the case of an AC motor driven pump, the impeller may turn in either of two directions depending on the phase angle of the AC power at the moment the motor is started. In order to provide predictable operation in an AC motor driven pump, the pump volute and impeller are specially designed so that the inlet and outlet of the pump are the same regardless of the rotational direction of the impeller.
Prior art FIGS. 1 and 2 illustrate a typical AC motor driven volute pump 10. The pump 10 includes an AC motor 12, a pump volute 14, and an impeller shaft 16 which is coupled to an impeller hub 18 having a plurality of radial vanes or fins 20. The pump volute 14 is symmetrical about a vertical axis A with an axial inlet port 22 and a radial outlet port 24. Below a horizontal axis B, the volute 14 is substantially cylindrical in shape, but above the horizontal axis B, the volute tapers with substantially tangential side walls 21, 23 to the radial outlet port 24. The impeller hub 18 is arranged axially with the inlet port 22. The impeller hub 18 has a smaller diameter than the inlet port 22 so that fluid entering the inlet port 22 is free to flow around the impeller hub and into the spaces between the vanes 20. Regardless of the direction of rotation of the impeller hub 18 and vanes 20, the vanes will create a centripetal force resulting in a low pressure condition at the impeller hub 18. This low pressure condition will draw fluid into the volute 14 via the inlet port 22. The fluid will enter the spaces between the vanes 20 whereupon it will be driven radially outward from the hub 18 in either a clockwise or counterclockwise flow, and tangentially along either side wall 21 or 23, depending on the direction of flow, to the outlet port 24. Since the outlet port 23 and the side walls 21, 23 are arranged symmetrically about the vertical axis A,the radially outward driven fluid will ultimately exit the volute 14 through the outlet port 24 regardless of the direction of impeller rotation. Although this design of the volute and the impeller provides for a predictable direction of fluid flow regardless of the direction of rotation of the impeller, it does have some significant disadvantages.
In virtually every pumping operation, the exiting fluid is subject to back pressure. Since the outlet port 24 in the pump 10 is arranged symmetrically relative to the impeller, fluid under back pressure is free to re-enter the volute 14 and be recirculated by the impeller. This is illustrated in FIG. 2 by the arrows which point down from the outlet port 24. As those skilled in the art will appreciate, the problem of recirculating back flow is endemic to AC motor drive pumps using symmetrical volutes. In DC motor driven pumps, as mentioned above, the outlet port can be arranged tangentially to the impeller in an asymmetrical volute so that backflow is virtually eliminated. The problem of backflow recirculation in AC motor driven pumps decreases their efficiency since the outlet volume and pressure decreases when fluid is being recirculated.